Digital Electronics Computer and Electronics Engineering Roger Sash Herb Detloff Alisa Gilmore
Digital Electronics Objectives: The objectives of this module are to: #
Become familiar with basic digital counter concepts
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Test counter circuits using circuit simulation software
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Build digital circuits using prototyping equipment
To accomplish the objectives, we will give you a short overview of electrical and digital logic concepts. You will then build circuits described later in the handout using the MultiSim simulation program. A variety of circuits are described; however, you may not have time to complete the simulation of all of them. The simulation program we will be using is MultiSim 8 by Electronics Workbench. www.electronicsworkbench.com. It is a user-friendly program which a user can create and test circuits in a relatively short time. It has a variety of components listed in a series of menus. One of the initial difficulties is finding the component you want. The circuits we are going to build start on page 11.
Simulating Circuits The circuits which we will construct and simulate are digital. In a digital circuit we are only concerned if a signal is present or not. A switch is an example of a digital circuit. It is either on or off. Terms used for the presence of a signal are on, high, true, and one. Terms used for the absence of a signal are off, low, false and zero.
Decimal Numbers in Computers
Many applications require the use of a counter. The next circuit will count from 0 to 9 in binary. The binary number system uses base 2. Characters in the right column (sometimes labeled a) have the value 20. In the next column to the left (labeled b) they have the value 21. The next column (labeled c) is 22. The next (d) is 23. They evaluate from left to right as 8 - 4 - 2 - 1. The binary numbers from zero through 9 are: Computer and Electronics Engineering
Binary
Decimal
dcba 8421 0000 0001 0010 0011 0100 0101 0110 0111 1000 1001
0 1 2 3 4 5 6 7 8 9
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For ease of use, we are using a modified binary system which only has values of zero through nine. This is referred to as binary coded decimal (BCD).
How to Find Components A list of component icons is shown along with the menu selections used to access them. The order of presentation is the same as the order in which you will be constructing the circuits. If your menu toolbar only shows icons without the text which identifies them, you need to turn on the text so the toolbar is as shown below:
How to turn on the Toolbar text
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Circuit 1: Toggle Flip-Flop
To flip the components so they are oriented as shown, use Edit | Orientation | Flip Horizontal
Circuit 1: Toggle Flip Flop
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Circuit 2: Four Bit Binary Counter1
Use COPY PASTE on Circuit 1 to make the four bit binary counter of Circuit 2. Add the proper connections between the flip-flops and add the digital display.
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Circuit 3 consists of a signal generator set to give one transition per second. The counter integrated circuit, 7490, counts the transitions in binary and the value is shown on the four LED displays. The binary signals need to be converted to drive a 7 segment display. The binary to 7 segment decoder performs this task.
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Circuit 3 – Binary Counter – 7 Segment Decoder – Display Circuit 8 – Binary Counter – 7 Segment Decoder – Resistors – 7 Segment Display
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Circuit 3:
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Locate using the same menues as the 74LS90N except select 74LS47N.
Double click on symbol to set values on the device. If the circuit is running when you change its speed, you will have to stop it and start it again for the new value to be recognized.
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Circuit 4. Two Decade Counter
Do a COPY PASTE of Circuit 3 to make the two decade counter of Circuit 4. Remove the count generator from the left digit. Connect the QD output of the right counter to the count input of the left counter. This will cause the 10’s digit to increment when the units digit goes from 9 back to 0.
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Circuit 4: Changing Page Size: Edit | Properties |
After you have completed the above circuits to your satisfaction, you can, as time permits, experiment with the remaining circuits. Next we will make a two decade counter which will count from 00 to 99. Change the size of the drawing sheet to size B. Move the circuit to the right side of the page by doing Edit | Select All (control A) and dragging the components to the desired position. Make a copy of the circuit by Select All and Paste (control V). Delete the signal generator on the left counter. The QD signal of the least significant digit will be used to clock the next stage. Place a wire from INA to QD.
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Create the following circuit. Use the DGND for the GND component. Use the DCD_HEX component which combines the decoding and display in one component. This will speed simulation so the circuit will count faster. The switch will cause the counter to reset when connected to Vcc and to count normally when connected to ground. Double click on the wire connected to R01 and name the net Reset.
Circuit 5. Counter with Display
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Circuit 5:
Naming a Wire
A warning you get if you have previously used the same name.
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Copy the circuit three times to make a four digit counter. Double click the wire connected to R01 and name it Reset. You should get a warning that that name has been used. Click OK. This will make a connection between both wire named Reset without having to draw the line. Do the same to the other two counters. Connect the QD of a counter to the INA of the next counter. Set the clock frequency to 1000Hx. Remember to stop and then start the simulation after changing the clock speed.
Circuit 6
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Use of the Breadboarding Trainers The trainers we will be using are a self-contained unit which has its own power source, switches, displays and an area in which to build prototype circuits. The integrated circuits have already been placed horizontally on the breadboard. This orientation places pin 1 on the lower left corner. The pin numbering progresses counter clockwise. The largest pin number will be on the top left corner. The 74LS08, 74LS32, and the 74LS90 have 14 pins. The 74LS47 has 16 pins. Jumper wires are placed in the holes adjacent to pins which are to be connected. The horizontal rows of pins adjacent to the red lines have been connected to +5 volts (Vcc). The horizontal rows of pins adjacent to the blue lines have been connected to ground. The figure below shows where the integrated circuits are installed on the breadboard. On the 74LS90, a wire connects pin 1 to pin 12, another wire connects pin 7 to the ground strip, and another connects pin 5 to the Vcc strip. The four holes which are vertical to a pin are connected together so it doesn’t matter which of the four holes is used. The upper row is used for the first circuit and the bottom row is used for the second circuit.
The automobile buzzer circuit and the counter circuit will be built on the prototyping boards. It can be confusing as to what has been wired and what still needs to be wired. A good method of keeping track of which wires have been connected is to put a check mark on the schematic or highlight wires as they are connected.
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To help keep track of which wires have been placed, place a checkmark on the wire or highlight the wire on the schematic.
This is the first part of the counter circuit you will build in the lab. Wire the 7490 and 7447. Connect the counter outputs to LEDs 4, 5, 6, & 7 on the trainer. Apply power and toggle the Pulse switch. The LEDs should count in binary from 0 through 9. When this is working properly, connect the remainder of the circuit as shown on the following page. A picture on page 17 shows how the completed circuit will look.
Circuit 7A BCD Counter on Trainer Computer and Electronics Engineering
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Circuit 7B – BCD Counter on Trainer Computer and Electronics Engineering
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References: Simulation Software: Electronics Workbench USA 60 Industrial Park, #068 Cheektowaga, NY 14227 www.electronicsworkbench.com Electronic Components: Jameco Electronic Components 1355 Shoreway Road Belmont, CA 94002 www.jameco.com Breadboard: PAD_234 Digital/Analog Trainer Electronix Express 365 Blair Road Avene l, New Jersey 07001 www.elexp.com/tst_234.htm
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